Link aggregation is a technique by which a group of parallel physical links between two endpoints in a data network can be joined together into a single logical link. Traffic transmitted between the endpoints is distributed among the physical links in a manner that is transparent to the clients that send and receive the traffic. Link aggregation offers benefits of increased bandwidth, as well as increased availability, since the logical link can continue to function (possibly with reduced bandwidth) even when one of the physical links fails or is taken out of service.
For Ethernet networks, link aggregation is defined by Clause 43 of IEEE Standard 802.3, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications (2002 Edition), which is incorporated herein by reference. Clause 43 defines a link aggregation protocol sublayer, which interfaces between the standard Media Access Control (MAC) layer functions of the physical links in a link aggregation group and the MAC clients that transmit and receive traffic over the aggregated links. The link aggregation sublayer comprises a distributor function, which distributes data frames submitted by MAC clients among the physical links in the group, and a collector function, which receives frames over the aggregated links and passes them to the appropriate MAC clients.
The 802.3 standard does not impose any particular distribution algorithm on the distributor, other than forbidding frame duplication and requiring that frame ordering be maintained over all frames in a given “conversation.” (Clause 1.4 of the standard defines a conversation as “a set of MAC frames transmitted from one end station to another, where all of the MAC frames form an ordered sequence, and where the communicating end stations require the ordering to be maintained among the set of MAC frames exchanged.”) In practice, this requirement means that the distributor must pass all frames in a given conversation to the same physical port, for transmission over the same physical link.
Annex 43A of the 802.3 standard, which is also incorporated herein by reference, describes possible distribution algorithms that meet the requirements of the standard, while providing some measure of load balancing among the physical links in the aggregation group. The algorithm may make use of information carried in each Ethernet frame in order to make its decision as to the physical port to which the frame should be sent. The frame information may be combined with other information associated with the frame, such as its reception port in the case of a MAC bridge. The information used to assign conversations to ports could thus include one or more of the following pieces of information:                a) Source MAC address        b) Destination MAC address        c) Reception port        d) Type of destination address        e) Ethernet Length/Type value        f) Higher layer protocol informationA hash function, for example may be applied to the selected information in order to generate a port number. Because conversations can vary greatly in length, however, it is difficult to select a hash function that will generate a uniform distribution of load across the set of ports for all traffic models.        
Service level agreements between network service providers and customers commonly specify a certain committed bandwidth, or committed information rate (CIR), which the service provider guarantees to provide to the customer at all times, regardless of bandwidth stress on the network. Additionally or alternatively, the agreement may specify an excess bandwidth, which is available to the customer when network traffic permits. The excess bandwidth is typically used by customers for lower-priority services, which do not require committed bandwidth. The network service provider may guarantee the customer a certain minimum excess bandwidth, or excess information rate (EIR), in order to avoid starvation of such services in case of bandwidth stress. In general, the bandwidth guaranteed by a service provider, referred to as the peak information rate (PIR), may include either CIR, or EIR, or both CIR and EIR (in which case PIR=CIR−EIR). The term “guaranteed bandwidth,” as used in the context of the present patent application and in the claims, includes all these types of guaranteed bandwidth.